1. Field of the Invention
The present invention relates to tools for testing earth formations in boreholes and more particularly for making formation pressure measurements, acquiring information concerning formation permeability and productivity, and retrieving samples of formation fluids.
2. Description of the Prior Art
Formation testing tools of the prior art of which I am aware have a number of deficiencies. It is important that such tools should have an effective fail-safe arrangement to assure that the parts that are extended into contact with the formation when the tool is set can be retracted in the event of power failure, so that the tool can be removed from the borehole. The fail-safe arrangements of the prior art that I know of are actuated by a tensioning of the tool suspension cable to shear a pin or the like, and are subject to problems such as unintentional shearing of the pin, or inability to exert the requisite tensioning force due to cable key seating.
Formation testing tools conventionally provide a pre-test chamber or chambers into which a small quantity of formation fluid (typically about 20 c.c.) can be drawn in order to make formation shut in pressure measurements and obtain indications of formation permeability and potential production. Once the pre-test procedure has been initiated, the entire pre-test chamber capacity must be filled with formation fluid before shut in pressure can be determined, which in the case of low permeability formations can consume considerable time. In addition, the lack of control between the initiation and completion of the pre-test procedure precludes desirable flexibility.
Formation testing tools are typically quite long, and a considerable portion of their length is in the sample chamber portion, which is conventionally rigidly attached below the seal pad. While the tool is set, or when attempting to free the tool, this sample chamber portion can be jammed against the wall of the borehole and become differentially stuck.
Formation testing tools of the prior art that I know of have had problems in maintaining isolation of the formation at the seal pad when testing in unconsolidated formations.
Patents that exemplify prior art formation testing tools are U.S. Pat. Nos. 3,811,321, 3,813,936, 3,858,445, 3,859,850, 3,859,851, 3,864,970, 3,924,468, and 3,952,588.